1. Field of the Invention
The present invention relates to a method for determining an exposure condition and a computer-readable storage medium that stores a program for determining an exposure condition.
2. Description of the Related Art
Exposure is the process most important in determining a line width in a circuit pattern of a semiconductor device in the manufacture of a semiconductor device. In exposure, an original is illuminated by an illumination optical system with light from a light source, an image of a pattern of the original is projected on a photosensitive material applied onto a substrate and exposure is made through a projection optical system. This processing is performed by an exposure apparatus.
Exposure apparatuses that use a technique for achieving higher resolution are being developed because circuits are required to have smaller line width compared with a wavelength of light of a light source used in exposure. Examples of parameters that determine resolution include a wavelength of light of a light source, a numerical aperture (NA) of a projection optical system, and a process-dependent numerical value, so-called a process factor k1. Examples of techniques for achieving higher resolution, as described above, include increasing NA and reducing k1. Examples of techniques for reducing k1 include modified illumination, polarized illumination, and adjustment of an aberration in a projection optical system.
The number of techniques for achieving high resolution is increasing and such techniques are becoming increasingly complicated. Therefore, in order to project a pattern onto a substrate and expose the substrate with high accuracy, it is desirable to appropriately set a condition for exposure (i.e., an exposure condition) by setting various parameters (e.g., the shape of illumination light (hereinafter referred to as an illumination shape), NA, aberration, the state of polarization of illumination light) by making full use of the above techniques.
One approach is a method for determining an illumination shape for optimally resolving a pattern by simulation. For example, Japanese Patent Laid-Open No. 6-120119 discloses a method that sets initial values of an optical condition (e.g., NA, wavelength, σ) and an illumination shape, calculates a pattern image on the surface of a substrate when a pattern of an original is projected on the substrate, and alters the illumination shape repeatedly until the pattern image is as close to a target pattern image as possible.
Japanese Patent Laid-Open No. 2005-26701 discloses a method for optimizing optical proximity correction and parameters for use in an exposure apparatus. Examples of the parameters for use in an exposure apparatus include NA and σ. Examples of the optical proximity correction include change of a line width in a designed pattern and addition of a supplement pattern that is not to be resolved.
Also in the method disclosed in this patent document, a pattern image to be projected on a substrate is calculated by simulation, as in Japanese Patent Laid-Open No. 6-120119, and exposure conditions including parameters for use in an exposure apparatus and optical proximity correction for a layout pattern are repeatedly modified until the pattern image to be projected on the substrate is as close to a target pattern image as possible.
The methods disclosed in both patent documents determine an exposure condition that enables a target pattern to be optimally reproduced on a substrate. This reduces the number of defects in resolving a pattern formed on the substrate.
Whether a produced device is defective or non-defective is determined by inspection of electrical characteristics of the device. Therefore, a device that has irregular line widths in an evaluation pattern formed on the substrate may be determined to be a non-defective product, and, on the contrary, a device that has a uniform line width in an evaluation pattern formed on the substrate may be determined to be a defective product. In other words, optimal reproduction of a target pattern on the substrate and manufacturing yield of devices do not completely match each other.
As a result, in order to enhance manufacturing yield of devices, it is necessary to consider electrical characteristics of devices to be produced in setting exposure conditions.